HORIZON INTERNATIONAL SOLUTIONS SITE

Palau's beautiful coral reefs are surprisingly resistant to the effects of ocean acidification.: Photograph courtesy of WHOI

As the ocean absorbs atmospheric carbon dioxide (CO₂) released by the burning of fossil fuels, its chemistry is changing. The CO₂ reacts with water molecules, lowering ocean pH (making it more acidic) in a process known as ocean acidification.

This process also removes carbonate, an essential ingredient needed by corals and other organisms to build their skeletons and shells.

Will some corals be able to adapt to these rapidly changing conditions? If so, what will these coral reefs look like in the future?

Now, in time for World Oceans Day on June 8, 2015 scientists are studying coral reefs in areas where low pH is naturally occurring to answer questions about ocean acidification, which threatens coral reef ecosystems worldwide.

Palau reefs dodge ocean acidification effects

One such place is Palau, an archipelago in the far western Pacific Ocean. The tropical, turquoise waters of Palau's Rock Islands are naturally more acidic due to a combination of biological activity and the long residence time of seawater in their maze of lagoons and inlets.

Seawater pH in Palau's Rock Island bays is as low as the open ocean may be at this century's end.: Photograph by Hannah Barkley courtesy of NSF.

Seawater pH within the Rock Island lagoons is as low now as the open ocean is projected to reach as a result of ocean acidification near the end of this century.

A new study led by scientists at the Woods Hole Oceanographic Institution (WHOI) found that coral reefs in Palau seem to be defying the odds, showing none of the predicted responses to low pH except for an increase in bioerosion--the physical breakdown of coral skeletons by boring organisms such as mollusks and worms.

A paper reporting the results is published on 5 June 2015 in the journal Science Advances: Changes in coral reef communities across a natural gradient in seawater pH. (The Abstract from this article is included below.)

"This research illustrates the value of comprehensive field studies," says David Garrison, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the research through NSF's Ocean Acidification (OA) Program. NSF OA is supported by the Directorates for Geosciences and for Biological Sciences.

Coral bioerosion, including these borings made by bivalves, increases during ocean acidification.: Photograph by Hannah Barkley courtesy of NSF.

"Contrary to laboratory findings," says Garrison, "it appears that the major effect of ocean acidification on Palau Rock Island corals is increased bioerosion rather than direct effects on coral species."

Adds lead paper author Hannah Barkley of WHOI, "Based on lab experiments and studies of other naturally low pH reef systems, this is the opposite of what we expected."

Photograph by Alexi Shalapyonok courtesy of NSF: Photograph by Alexi Shalapyonok courtesy of NSF

Experiments measuring corals' responses to a variety of low pH conditions have shown a range of negative effects, such as fewer varieties of corals, more algae growth, lower rates of calcium carbonate production (growth), and juvenile corals that have difficulty constructing skeletons.

"Surprisingly, in Palau where the pH is lowest, we see a coral community that hosts more species and has greater coral cover than in the sites where pH is normal," says Anne Cohen, co-author of the paper.

"That's not to say the coral community is thriving because of the low pH, rather it is thriving despite the low pH, and we need to understand how."

When the researchers compared the communities found on Palau's reefs with those in other reefs where pH is naturally low, they found increased bioerosion was the only common feature.

"Our study revealed increased bioerosion to be the only consistent community response, as other signs of ecosystem health varied at different locations," Barkley says.

The riddle of resilience

How do Palau's low pH reefs thrive despite significantly higher levels of bioerosion?

The researchers aren't certain yet, but hope to answer that question in future studies.

They also don't completely understand why conditions created by ocean acidification seem to favor bioeroding organisms.

One theory--that skeletons grown under more acidic conditions are less dense, making them easier for bioeroding organisms to penetrate--is not the case on Palau, Barkley says, "because we don't see a correlation between skeletal density and pH."

Coral reefs under low pH conditions in Palau are still diverse and have high coral cover.: Photograph by Hannah Barkley courtesy of NSF.

Though coral reefs cover less than one percent of the ocean, these diverse ecosystems are home to at least a quarter of all marine life. In addition to sustaining fisheries that feed hundreds of millions of people around the world, coral reefs protect thousands of acres of coastlines from waves, storms and tsunamis.

Huge Porites corals are relatively healthy throughout Palau waters.: Photograph by Hannah Barkley courtesy of NSF.

"On the one hand, the results of this study are optimistic," Cohen says. "Even though many experiments and other studies of naturally low pH reefs show that ocean acidification negatively affects calcium carbonate production, as well as coral diversity and cover, we are not seeing that on Palau.

"That gives us hope that some coral reefs--even if it is a very small percentage--might be able to withstand future levels of ocean acidification."

Along with Barkley and Cohen, the team included Yimnang Golbuu of the Palau International Coral Reef Center, Thomas DeCarlo and Victoria Starczak of WHOI, and Kathryn Shamberger of Texas A&M University.

The Dalio Foundation, Inc., The Tiffany & Co. Foundation, The Nature Conservancy and the WHOI Access to the Sea Fund provided additional funding for this work.

This news is from the National Science Foundation June 5, 2015. This article was published on the Horizon International Solutions Site on 7 June, 2015.

Resources:

Horizon International coral reefs website: http://www.magicporthole.org

NSF Related Websites:

NSF News: Palau's coral reefs surprisingly resistant to ocean acidification: http://www.nsf.gov/news/news_summ.jsp?cntn_id=130129
NSF Grant: Toward Predicting the Impact of Ocean Acidification on Net Calcification by a Broad Range of Coral Reef Ecosystems: Identifying Patterns and Underlying Causes: http://www.nsf.gov/awardsearch/showAward?AWD_ID=1220529&HistoricalAwards=false
NSF News: Global Warming Refuge Discovered Near At-Risk Pacific Island Nation of Kiribati: http://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=123999&preview=false
NSF News: Ocean Acidification: NSF awards $11.4 million in new grants to study effects on marine ecosystems: http://www.nsf.gov/news/news_summ.jsp?cntn_id=132548

About the National Science Foundation:

The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2015, its budget is $7.3 billion. NSF funds reach all 50 states through grants to nearly 2,000 colleges, universities and other institutions. Each year, NSF receives about 48,000 competitive proposals for funding, and makes about 11,000 new funding awards. NSF also awards about $626 million in professional and service contracts yearly.